Data storage devices used in computer systems include hard disks and heads which fly over the disk. Generally, the heads flying height is in the range of six to ten millionths of an inch above the surface of the disk. Since the head flies close to the disk at high velocity it is important that the disk is level or smooth. High points and other changes in the topography on the disk surface increase the risk of a head crash. A head crash is an unintended contact of the head with the disk which results in loss of data.
To prevent undesirable head crashes, disks are measured for smoothness with a Runout Velocity and Acceleration ("RVA") test.
Runout, as is well-known in the art, is the smoothness of the disk or the amount the displacement varies from a fixed probe to the disk as the probe travels over the disk. Runout is also known as the circumferential profile of the disk. In other words, when a probe is passed over a disk in a spiral fashion similar to a phonorecord, the distance the fixed probe is away from the disk (displacement) can be plotted against the time it takes the fixed probe to move over the spiral path. In a sense, the spiral path can be "unwound" and the plot of displacement from a fixed point with respect to time can be plotted as a continuous function.
As is well-known from physics and calculus, the first derivative of a functional relationship is the change in displacement with respect to time. This is known as velocity and gives information regarding the direction of the displacement. In addition, when the first derivative is zero it generally indicates that there is a maxima or minima occurring at that time. In this instance, a maxima or minima would correspond to a bump on the disk or a valley between bumps on the disk.
The second derivative of the function of displacement as a function of time gives further information about the maxima or minima or, in this instance, the bumps and valleys on the surface of the disk. From the second derivative it is well-known and easily found whether the point is a bump or a valley. In addition, the acceleration also gives an indication of the severity of a particular bump on a disk. The second derivative is also used to find the radius of curvature of the bump which determines the severity of the bump. Finding the second derivative of displacement with respect to time is also known as the acceleration.
The acceleration associated with a pump or valley is very important to the specific situation of a head flying over a disk. The head, ideally, flies a fixed distance off the disk. If a disk were perfectly smooth (i.e., absolutely devoid of any bumps or valleys) the head would fly a level path. However, disks are not perfect and the head must change course to maintain the fixed distance above the surface. For example, the head must dive or dip to fly into and out of a valley and must climb to fly over a hill or mountain. To enable the head to change course (i.e., climb or dive) a force must be applied to the head. A force equals mass times acceleration. Since the mass is constant, the head must be accelerated upward or downward to apply the force necessary to cause the head to climb or dive. In order for the head to stay at a constant height above a disk surface, the head must be accelerated at essentially the same acceleration as those associated with the bumps or valleys. In other words, the acceleration associated with a bump or valley on a disk must essentially be matched by the head in order for the head to fly at an essentially fixed height over a disk.
A physical limitation of the head is that it has a limited capability in acceleration. In other words, it can only change course up and down so fast.
If the acceleration associated with a bump or a valley on a disk is higher than the maximum acceleration the head is capable of, then it means one of two things. If a bump is encountered on the disk, the head will be unable to climb fast enough and the head will crash into the bump (a head crash). If a valley is encountered on the disk the head will be unable to drive fast enough and the head will skim over the valley. Data loss results since the head will fly more highly than it is supposed to. For the sake of simplicity, bumps and valleys are spoken of in terms of the acceleration associated with them.
In present Runout Velocity and Acceleration testers, capacitance probes pass over the disk at a height of about ten thousandths (0.010") of an inch. The probes and circuitry measure the second derivative of the circumferential disk profile which indicates curvatures in terms of accelerations.
Presently, two methods of RVA testing exist. A spiral method is used as a go-no/go gauge in manufacturing. The probes are moved in toward the center of a rotating disk. The analog readout of the RVA tester is fed to a comparator having a preset level therein. If the analog readout equals or exceeds the preset level the comparator signals a rejection of the disk.
A digital method tests one track of the magnetic disk at a time. A pair of probes are passed over a particular track for several revolutions. The probes measure the greatest change in height profile (acceleration) for each revolution, digitize and store the data. The digitized highest acceleration values are then averaged for the particular track to give a measure of disk quality.
Both of these methods of RVA testing have disadvantages. The digital method requires up to fifteen minutes to test a disk, which is too time-consuming for commercial production testing. Hence, the digital method is used only to test a small percentage of manufactured disks. Also, the track-to-track digital test method yields varying results when the test is repeated on the same disk.
The spiral method of RVA testing is faster and is used in manufacturing, however, the test is incomplete. The spiral method fails to provide quantifiable or verifiable data. In addition, the spiral method is not as accurate as track-to-track digital method.
Thus, there is a need for an RVA tester capable of being used in manufacturing to produce repeatable, verifiable and quantifiable data for the highest acceleration on a particular disk.